This week in Las Vegas, the new management team running Yahoo — including CEO Marissa Mayer — is at International CES to schmooze with big advertisers and convince them that Yahoo is the place to put large chunks of their marketing budgets.

One of the longtime selling points of the company is the sheer size of its audience, especially for the key money-making parts of the site — the homepage, Yahoo Mail and search.

But private stats from comScore show that those three areas have continued their longtime decline over the last year, in some cases dropping significantly. In November and December, for example, compared to the same two months a year ago, U.S. search was down 28 percent and 24 percent respectively, while mail was down 16 percent and 12 percent.

This matters a great deal, since the troika of homepage, mail and search have been the critical driver of the Yahoo value ecosystem for advertisers.

The impact of those drops is felt all over Yahoo, whose music, movie, games and travel site have also seen massive drop-offs in traffic year over year in those same months.

Stopping the decline is critical for Yahoo, since Mayer herself has underscored the need for size in her pushing for new businesses at Yahoo that are 100 million users in size and/or have revenue prospects of at least $100 million.

While this is a lofty vision, the reality of traffic falloffs on key properties is a vexing issue, especially since they remain its main source of revenue and also an important element in launching future products Mayer is promising will turbocharge the company.

It’s not that Yahoo is not huge, especially compared to most sites on the Web.

As one of the top Internet brands, according to a recent Nielsen report, the average number of total monthly unique visitors for the longtime Silicon Valley Internet company in 2012 was 141.6 million, No. 3 behind Google and Facebook in the U.S. market. Similar rankings were reported by comScore, which placed Yahoo at the No. 2 spot after Google, with 171.4 million monthly visitors in November.

But, for many years, traffic to those important consumer destinations of Yahoo has been on a clear and unstopping decline, statistics (usually from comScore) that the company nonetheless always dutifully puts in its earnings slides — see below — for investors to get some idea of the major and vexing issues facing the company.

That was suddenly ended in the last quarter with the engagement slide removed from Yahoo’s public deck entirely. Not all companies include such stats, so when I inquired as to why the company had made the change, Yahoo PR never returned my phone call.

But it’s not hard to guess the reason for the shift — the numbers were not good and they called more attention to Yahoo’s glaring challenge, which is getting users reengaged with its products by creating what Mayer has dubbed several times “delightful” experiences.

According to numerous sources, that has also been the case within the company too, with the new regime restricting an internal transparency initiative pushed by former Chief Product Officer Blake Irving that shared product performance numbers with the top 100 leaders at Yahoo.

And while it’s an interesting strategic choice, several sources inside the company this week urged me to get ahold of increasingly worrisome numbers from comScore — available to its private clients — comparing November 2011 to November 2012 and also December 2011 to December 2012 at home and work in the U.S.

So I did, getting the same stats from numerous sources — numbers that a spokesman for comScore confirmed were correct.

And, as promised, they are worrisome indeed.

In November 2012, compared to November 2011, the monthly unique visitors to the homepage declined 17 percent to 91.8 million from 110.9 million; Yahoo Mail dropped 16 percent (from 92 million to 77.7 million); and Yahoo search dropped 28 percent (from 93.3 million to 66.9 million).

Also off significantly for all three areas, often by one-third, were a plethora of other stats: Percentage of reach, total minutes, total page views, total visits and more.

One of the only bright spots for Yahoo was the relatively small Flickr sites, which were up 37 percent — 26.7 million versus 19.4 million — in unique monthly visitors year over year. The photo-sharing site — which has been getting a much-needed refresh — was also up in all other stats.

But Flickr — which Mayer (pictured here) has laudably touted and supported after years of inexplicable neglect — is not a money-maker for Yahoo, even if its return does burnish the company’s tech and innovation cred.

In December 2011 to December 2012, the homepage was more stable, gaining four percent in monthly uniques from 109.4 million to 114.2 million, but with other key stats both rising and falling. Total visits were up 14 percent, for example, while average minutes per visit was down 13.6 percent.

But the trouble for mail or search continued, off 12 percent (89.9 million to 78.7 million) and 24 percent (88.7 million to 67.4 million) respectively in monthly uniques, with similarly major declines in all other stats.

Mail recently got a refresh too under Mayer, despite some recent security glitches, so new stats will show if that will help stem the declines. Search is another story all together, with Yahoo in what can only be described as a dysfunctional partnership with Microsoft that numerous sources tell me Mayer is seeking to end.

The homepage, too, is undergoing a redo, with a design that has a decidedly more mobile and social feel, and pushing an ethos of Yahoo becoming a hub for content discovery. It is hoped the new look will boost traffic relatively quickly from its current downward trajectory.

To be fair, there can be lots and lots of reasons for these declines, although most of Yahoo’s competitors are, at worse, seeing a flattening of growth and not outright declines.

And sometimes Internet sites complain that services like comScore undercount, although Yahoo had previously used the firm in its public documents. More to the point, as multiple sources within the company note, the stats are directionally correct in that they closely track with internal Yahoo numbers.

Which is to say, traffic is going down rather than growing. That is clearly why Mayer has loudly stressed mobile since arriving at Yahoo, an area not included in these numbers that many sources said has strong growth to about 70 million monthly unique visitors via its apps and mobile-enabled Web offerings.

But unlike the homepage, mail and search — which push and pull traffic all over Yahoo and are responsible for most of its current monetization — mobile also makes very little money now. And Yahoo — unlike Facebook, which recently did — does not break out mobile results.

So, it will be interesting to see if the company does so when it reports fourth-quarter earnings on January 28 and also if it says anything about continued traffic declines of its traditional Web business in the period and the impact on revenue.

Still, there are lots of ways to counter declining or flat revenues, even with declining traffic — via cost cuts, efficiencies, charging more and selling assets (as Yahoo did in the last quarter). And Yahoo has ably managed to keep its operating margins growing over the years, despite both the declines in traffic and moribund growth in its revenue.

But the real and only fix is the drastic fix to existing tentpoles Yahoo has and the creation or acquisition of products that excite consumers and, therefore, advertisers.

It’s not an easy thing, of course, as well-known venture capitalist Ben Horowitz recently wrote in his blog about the need to focus on products over building and improving culture — one of Mayer’s other big initiatives at Yahoo.

Wrote Horowitz in what I consider one of the clearest articulations of what it takes to win for startups, as well as big companies like Yahoo:

“The primary thing that any technology startup must do is build a product that’s at least 10 times better at doing something than the current prevailing way of doing that thing. Two or three times better will not be good enough to get people to switch to the new thing fast enough or in large enough volume to matter. The second thing that any technology startup must do is to take the market. If it’s possible to do something 10X better, it’s also possible that you won’t be the only company to figure that out. Therefore, you must take the market before somebody else does.”

If you want to take a gander, here are some more of those old Yahoo quarterly engagement slides, which were recently eliminated from its presentations:

(Note: I reached out to Yahoo’s outside PR firm — since they do respond to queries — and also some company execs to get a comment on this story, but so far there has been none.)

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